This invention relates generally to gas turbine engines and, more particularly, to methods and apparatus for controlling combustor operability.
Gas turbine engines operate in many different operating conditions, and combustor performance facilitates engine operation over a wide range of engine operating conditions. More specifically, stable combustion facilitates preventing engine blowout and enables the engine to operate at engine rated thrust and/or power levels. Further, for gas turbine engines operated with dry low oxides of nitrogen (DLN) techniques, combustion stability also facilitates controlling oxides of nitrogen (NOx) and carbon monoxide (CO) emissions.
At least some known DLN combustion systems utilize premixed fuel and air, and operate with lean fuel/air (F/A) ratios to facilitate reducing NOx emissions. Lean fuel/air ratios are defined as operations wherein the ratio of fuel to air supplied to the combustion chamber is below a stoichiometric ratio of fuel to air required for the fuel under consideration. However, one possible consequence of the lean, premixed operation is that the combustion system may operate near a lean blow out (LBO) boundary. Lean blow out, or weak extinction, is the operating point at which the mixture of fuel and air is no longer flammable. For premixed multi-nozzle systems, weak extinction can be defined as the operating point at which there is a significant drop in the combustion efficiency and/or complete extinction of the flame. The LBO boundary or constraint, if violated, may result in partial or complete blowout (i.e., loss of the combustion flame).
In some conventional DLN fuel nozzles, a mixing potential is reduced to produce locally richer regions with improved flammability for acceptable combustion dynamics and lean blow out (LBO) limits. However, the higher adiabatic flame temperatures of these locally richer regions produce a significant or the majority of the combustor's NOx. The rate of NOx formation increases exponentially with temperature, so even a small region of higher temperature can produce a large percentage of the total NOx emissions.